US president Barack Obama is one of many who paid tribute to Williams this morning.

In a statement released by the White House, Obama said: "Robin Williams was an airman, a doctor, a genie, a nanny, a president, a professor, a bangarang Peter Pan, and everything in between. But he was one of a kind. He arrived in our lives as an alien – but he ended up touching every element of the human spirit."

Emergency workers arrived five minutes later and pronounced the man dead. He was identified as 63-year-old Robin McLaurin Williams.

An investigation is now underway but, in the statement, the Sheriff’s Office said it "suspects the death to be a suicide due to asphyxia".

Legendary director Steven Spielberg, a close friend of Williams, also paid tribute to him. "Robin was a lightning storm of comic genius and our laughter was the thunder that sustained him," Spielberg said in a statement cited by Variety.

"He was a pal and I can’t believe he’s gone," added Spielberg, who famously phoned Williams to cheer himself up during filming of his harrowing 1993 Holocaust drama Schindler’s List.

Australian director George Miller, who worked with Williams on Happy Feet and Happy Feet 2, described him as a genius actor and comic.

"There was a quality in the way that his mind worked so quickly and was able to construct brilliant tour de force pieces, unbidden as it were and perfectly formed," he said. "That was an extraordinary thing to watch.

"And what people may not know was that he was also a magnificent human being. He had a huge heart and that intelligence you saw in his humour was in his normal conversation as well. He could range so widely and deeply on every single topic you could imagine. It was quite breathtaking.

"So in every way, it will be a deeply felt absence."

Miller said Williams worked off a script in the Happy Feet movies but would improvise brilliantly.

"He would go off on his interpretations. It was like comic jazz and without exception he would come up with something better. You’d just let him go and how it came out so perfectly was extraordinary."

From Mork to Oscar glory

Williams rose to fame in the late 1970s after he broke through as a guest star on the iconic sitcom Happy Days, playing the alien Mork. That role won him his own spin-off series, Mork & Mindy.

Williams' co-star on the hit sitcom, Pam Dawber, expressed her grief today. "I am completely and totally devastated," she said in a statement. "What more can be said?"

Williams won an Academy Award for best supporting actor in the 1997 film Good Will Hunting, in which he played a psychologist.

He most recently starred in the CBS comedy The Crazy Ones, which was cancelled after one season. His co-star Sarah Michelle Gellar has tweeted pictures of them together as a tribute.

In an interview with Parade magazine last year, Williams said that regular TV work appealed to him because he had lost a lot of money through two divorces.

"Divorce is expensive," he said. "I used to joke they were going to call it 'all the money,' but they changed it to 'alimony.' It's ripping your heart out through your wallet.

"The idea of having a steady job is appealing ... there are bills to pay. My life has downsized, in a good way. I’m selling [my $US35 million dollar home] in Napa. I just can’t afford it anymore."

Williams – who said he was "shy" as a child until he began studying drama – was renowned for his quick wit and gregarious demeanour. In 1973, he was accepted into the Juilliard Academy, where he had several classes in which he and the late Christopher Reeve were the only students and John Houseman was the teacher.

Encouraged by Houseman to pursue comedy, Williams identified with the wildest and angriest of performers: Jonathan Winters, Lenny Bruce, Richard Pryor, George Carlin.

"You look at the world and see how scary it can be sometimes and still try to deal with the fear," he told the Associated Press in 1989. "Comedy can deal with the fear and still not paralyse you or tell you that it’s going away. You say, OK, you got certain choices here, you can laugh at them and then once you’ve laughed at them and you have expunged the demon, now you can deal with them. That’s what I do when I do my act."

His most popular films include Good Morning Vietnam (1987), in which he played an army radio DJ; Dead Poets Society (1989); Mrs Doubtfire (1993); The Birdcage (1996), in which he played a gay nightclub owner; and Patch Adams (1998), a biographical comedy about a doctor who uses humour as medicine.

He was last seen in The Face of Love with Annette Bening. His final films include a voice role in the British comedy Absolutely Anything, due to be released next year; Merry Friggin' Christmas, scheduled for November; and a Night At The Museum: Secret of the Tomb in December. He also starred in Boulevard, which debuted at the Tribeca Film Festival but has not been widely released yet.

A sequel to Mrs Doubtfire had also been announced, with filming expected to begin as early as this year.

Williams collected four Grammy Awards over the years and an Emmy for two television specials.

Comedian and actor Billy Crystal once observed that following Williams on stage was "like trying to top the Civil War".

Williams has spoken openly about his struggles with cocaine and alcohol addiction as a young man. In a stand-up show in Melbourne in 2010, he joked: ''I checked into a centre in wine country, just to hedge my bets.''

Earlier this year, he checked into a renewal facility to "focus on his ongoing sobriety", according to the Hollywood Reporter. This followed a relapse into alcoholism in 2006.

Williams was among the last to see John Belushi before the Saturday Night Live star died of a drug overdose in 1982.

He had been married three times. In 1978, he wed Valerie Velardi, with whom he had a son, Zachary Pym.

In 1989, he married Zachary’s nanny Marsha Garces, who was seven months pregnant with his daughter, Zelda Rae. The couple had a son, Cody Alan, two years later.